A type of gap in transmission line in the prior art is termed a “Mobius gap” due to its similarity to the connection in a strip of material that is applied to form a Mobius loop. Specifically to form a Mobius loop, as is known in the prior art, a single twist in made in the strip of material having a first side and a second side, a long slender strip of paper for example, and the two ends of the strip of material are butted together to form a loop. When the two ends of the strip are butted together in this fashion, the first side of the strip at the first end aligns with the second side of the strip at the second end such that if a pencil line is drawn across the butted connection, it would mark the first side of the strip on one side of the connection and the second side of the strip on the other side of the connection. If the pencil line is then continued along the strip without lifting the pencil from the strip, it is found that the pencil marks a continuous line on both sides of the strip forming the loop indicating that the connection of the two ends of the strip in the manner noted has resulted in the loop thus formed having only a single continuous surface. Specifically, the strip forming this loop no longer has a first side and a second side, but only a single side. This is the Mobius loop configuration, and the connection used to form the Mobius loop in the original long slender strip of material is termed a “Mobius connection.”
A Mobius-type connection may also be applied to a transmission-line structure, such as a length of coaxial cable for example, as is also known in the art. Consider a conventional coaxial transmission-line section having an outer conductor and an inner conductor and having a first end and a second end. The two ends of this coaxial transmission-line section are brought toward each other as would be done in a simple butt connection to form a loop. However, rather than a simple butt connection, the inner conductor at the first end is connected to the outer conductor at the second end, and the inner conductor at the second end is connected to the outer conductor at the first end. If a continuous electrical path is now traced, for example starting at the inner conductor at the first end and moving along the inner conductor from the first end, it is found that there is only a single conductor forming the loop. Specifically, starting at the inner conductor at the first end of the coaxial transmission-line section, the path travels continuously along the inner conductor of the line section until it reaches the second end of the line section at which point the path communicates unbroken to the outer conductor of the line section at the first end of the line section and proceeds along the outer conductor until it again reaches the second end where it communicates to the inner conductor of the first end of the line section, which is the starting point of the circuit path. Accordingly, as in the Mobius loop, where the two surfaces of a strip of material become a single surface with the Mobius connection, the two conductors of the coaxial transmission-line section become a single continuous conductor with the application of the Mobius connection. The connection of the two ends of the coaxial transmission-line section as described hereinabove is therefore also termed a Mobius connection when applied to the coaxial transmission-line section. However, since the coaxial cable inner conductors and outer conductors cannot be as gracefully connected in a Mobius connection as can be the ends of the strip in a Mobius loop applied to a strip of material, a small gap occurs at the point of the Mobius connection in the coaxial transmission-line section. This gap at the point of the Mobius connection of the coaxial transmission line section is termed a “Mobius gap”, in the prior art.
The Mobius gap is common in the prior art to provide a 1:1 ratio inverting transformer in a coaxial transmission-line section. A typical example of such a 1:1 inverting transformer is the Model 5100 Boradband Pulse Inverter by Picosecond Pulse Labs. Such a 1:1 inverting transformer is formed in a coaxial transmission line section comprising an outer conductor and an inner conductor further comprising a first end and a second end. At both the first end and second end of the coaxial transmission-line section the outer conductor is connected to ground, and the inner conductor is connected at the first end to a source and the inner conductor at the second end is connected to a load. In this configuration, the signal introduced at the source is passed substantially undisturbed to the load. To form a 1:1 inverting transformer, the coaxial transmission-line section is cut at a point between the first and second ends, and rejoined with a Mobius gap as described hereinabove. With the Mobius gap provided in the coaxial transmission-line section, the signal introduced at the first end of the coaxial transmission-line section is presented to the load at the second end with the same magnitude but inverted in sign. Therefore, the signal from the source is inverted when it is presented at the load.
At low frequencies, the coaxial transmission-line section comprising a Mobius gap appears as a short circuit to the source since the inner conductor at the first end of the coaxial transmission-line section comprising a Mobius gap eventually communicates to ground at the second end of the coaxial transmission-line section. For high-frequency signals, short pulses for example, where the coaxial transmission-line section is long with respect to the characteristic wavelength of the signal, the coaxial transmission-line section comprising a Mobius gap presents as a high-performance 1:1 inverting transformer. For example, if a square pulse is applied to the first end of a transmission-line section comprising a Mobius gap, and where the pulse width is shorter than the transit time of the coaxial transmission-line section, the pulse will travel along the coaxial transmission-line section, across the Mobius gap, continue along the coaxial transmission-line section being finally delivered to a load connected to the second end of the coaxial transmission-line section, and where the pulse when delivered to the load is inverted with respect to the polarity launched at the first end of the coaxial transmission-line section. Because the coaxial transmission-line section is long with respect to the pulse width, the connection to ground at the second end of the coaxial transmission-line section does not affect the source since there is insufficient time during the pulse for signals to travel the full length of the coaxial transmission-line section.
As noted hereinabove, the Picosecond Pulse Labs Model 5100 Broadband Pulse Inverting Transformer provides means to invert an RF signal and provides a 1:1 impedance transformation. A serious disadvantage of the Broadband Pulse Inverting Transformer taught by Picosecond Pulse Labs is that it is limited to a 1:1 impedance transformation ratio. Another serious disadvantage of the Broadband Pulse Inverting Transformer taught by Picosecond Pulse Labs is that it provides only an unbalanced, single-ended signal.
The earliest reference to the Mobius connection applied to a transmission line, and specifically to a coaxial transmission line, that could be located is in the paper “Characteristics of the Mobius Strip Loop,” Sensor and Simulation Note 7, 1964, by Carl E. Baum (“the noted paper”). A copy of the noted paper is attached for reference.
The sensor configuration described in the noted paper was termed “Mobius Strip Loop” because of the Mobius connection made at the gap were the outer and inner conductors of the transmission line comprising the sensor are cross coupled as shown in FIG. 4 of the noted paper.
Whereas the gap device of the Mobius Strip Loop sensor creates a Mobius-type structure in a coaxial transmission line similar to a Mobius connection made in a strip of flexible material, that gap device has become known as a “Mobius Gap.” When the term “Mobius Gap” is encountered by one skilled in the art of wide-bandwidth electromagnetic sensors, such as the Mobius Gap Loop for example, it is widely understood that such reference describes the gap device as shown in FIG. 4 of the noted paper.
FIG. 4 of the noted paper shows the Mobius Gap device as described by Gruchalla in Patent Application US 2007/0075802 A1 published Apr. 5, 2007.
Wide-bandwidth transformer devices are very common in the prior art for such applications as providing impedance matching between the source and load in radio-frequency (“RF”) applications. Balanced transformer devices (“balun”) are also common in applications where a balanced signal is required from a single-ended source and where a balanced signal is to be delivered to a single-ended load. In the prior art, it is problematic to provide both impedance transformation between two arbitrary impedances and single-ended-to-balanced transformation. Specifically, low-loss transformation of impedances is typically limited to ratios related by the squares of whole numbers. The following examples are easily provided with devices of the prior art: a 1:1 transformation, the square of 1, and a 4:1 transformation, the square of 2. However, a transformation such as 50 ohms to 100 ohms, an impedance ratio of the square-root of 2, is not typically provided in low-loss devices of the prior art. In prior-art devices providing such a transformation, bandwidth is limited to only several octaves and insertion loss is comparatively high. The devices of prior art cannot satisfy the requirements to provide transformation between single-ended and balanced circuits in a device that also provides impedance matching between two arbitrary impedances over a very wide-bandwidth and with very low loss.
Transformer devices providing 1:1 impedance matching between single-ended and balanced circuits are very common in the prior art. Such a single-ended to balanced 1:1 impedance-transformation device is described in U.S. Pat. No. 3,913,037, entitled “Broad Band Balanced Modulator,” to Yusaku Himono, et al. Yusaku teaches a configuration comprising as an integral element a transformer structure providing single-ended to balanced transformation and 1:1 impedance transformation, Item 8 and Item 2 according to Yusaku. According to Yusaku, a parallel-wire transmission line is wound about a toroidal magnetic core assembly thereby providing transition from a single-ended to a balanced configuration. A serious disadvantage of the prior art taught by Yusaku is that only a 1:1 impedance transformation is provided. Another serious disadvantage of the prior art taught by Yusaku is that its construction is generally limited to parallel-wire transmission-line sections. Such transmission line constructions are not totally bounded-wave electromagnetic configurations and therefore are severely limited in maximum operating frequency where the length of such line structure is comparatively long or where such line section is in the vicinity of other circuit elements or physical features of the system in which incorporated.
Wide-bandwidth impedance transformation devices where the transformation ratio is the square of whole numbers are very common in the prior art. Such transformation devices are classically termed in the prior art “constant-delay” transformers. A balanced transformation device for providing a 4:1 single-ended to balanced impedance transformation, an impedance transformation of 2 squared, is described in U.S. Pat. No. 2,231,152, entitled “Arrangement for Resistance Transformation,” to Werner Buschbeck. Buschbeck teaches a configuration of two coaxial transmission-line sections of equal impedance and equal electrical length connected in cross-coupled parallel at one end and in series at the other end where series and parallel connection refer here specifically to the effective arrangement of the line impedances and not to the line lengths. At the cross-coupled-connected end of the configuration taught by Buschbeck, the shields and center conductors of the two coaxial transmission-line sections are cross connected wherein the center conductor of each coaxial transmission-line section is connected to the shield conductor of the opposite coaxial transmission-line section. This arrangement effectively ties the impedances of the two coaxial transmission-line sections in parallel. Therefore, the impedance presented at this parallel connection of the two coaxial transmission-line sections is one half the impedance of the coaxial transmission-line sections. At the series-connected end of the configuration taught by Buschbeck, the shield conductors of the two coaxial transmission-line sections are series connected wherein the shield conductor of each coaxial transmission-line section is connected to the shield conductor of the opposite coaxial transmission-line section and the signal is taken from the two coaxial-line center conductors. This arrangement effectively ties the impedances of the two coaxial transmission-line sections in series. Therefore, the impedance presented at this series connection of the two coaxial transmission-line sections is twice the impedance of each coaxial transmission-line section. Accordingly, the impedance transformation between the parallel-connected feature and the series-connected feature in the prior art taught by Buschbeck is 4:1. Buschbeck additionally teaches ¼-wavelength means to control electromagnetic radiation from the excited shield conductors at the parallel-connected feature. A serious disadvantage of the prior art taught by Buschbeck is that only a 4:1 impedance transformation is provided, for example, 50 ohms to 200 ohms or 100 ohms to 25 ohms. Another serious disadvantage of the prior art taught by Buschbeck is that it must be applied where the various feature lengths are ¼ wavelength. Accordingly, the prior art taught by Buschbeck is limited to effectively single-frequency or very narrow-band operation.
A classic 4:1 impedance matching single-ended-to-balanced transformation device comprising coaxial transmission-line sections is the “Guanella Balun.” The Guanella balun is described in the article entitled “Novel Matching Systems for High Frequencies,” Brown-Boveri Review, Vol. 31, Sep. 1944, pp. 327-329, by Geanelli Guanella. Guanella teaches a configuration wherein the electrical arrangement is identical to the prior art taught by Buschbeck but with a magnetic core means introduced to improve the operating bandwidth. Whereas the device taught by Guanella is substantially electrically equivalent to that taught by Buschbeck, the device taught by Guanella is also limited to impedance transformation values that are the squares of whole numbers, 1:1 and 4:1 for example. This is a serious deficiency where matching of impedances having arbitrary impedance ratios is required.
Wide-bandwidth transformation devices providing transformation ratios other than the squares of whole numbers are also common in the prior art. Such devices are described in the article by Jerry Sevick entitled “Design and Realization of Broadband Transmission Line Matching Transformers,” Emerging Practices in Technology, EEE Standards Press, 1993. Sevick teaches an equal-delay transformer comprising series/parallel connections of several equal-length transmission-line sections of specific characteristic impedance to effect impedance transformation ratios other than the square of a whole number. As noted previously, these are termed constant-delay transformers in the art. For example, one configuration taught by Sevick comprises three 33.33-ohm transmission-line sections combined in series and parallel combinations in combination with magnetic core elements to provide a 2.25:1 transformation and wide-bandwidth performance. A serious deficiency of the prior art taught by Sevick is that the physical geometry does not present a balanced coupling to free space and therefore cannot provide high-performance balanced operation because of the single-ended parasitic free-space coupling.
In the same work referenced hereinabove entitled “Design and Realization of Broadband Transmission Line Matching Transformers,” Sevick also teaches a configuration providing improved balance with a 2.25:1 impedance-transformation ratio. This configuration taught by Sevick comprises a quadrifilar-wound transformer providing a 2.25:1 impedance transformation followed by a bifilar-wound Guanella 1:4 balun. The resulting configuration provides a 1:2.25 impedance transformation and balanced operation at the high-impedance port. Matching between, for example, a 50-ohm single-ended circuit and a 112.5-ohm balanced circuit is thereby provided. A serious deficiency in the prior art taught by Sevick is that the quadrifilar and bifilar winding configurations are not well defined in impedance and are not fully bounded-wave electromagnetic structures. Therefore, the configuration taught by Sevick is severely limited in operating frequency where the line lengths are comparatively long or where such line sections are in the vicinity of other circuit elements or physical features of the system in which incorporated.
It is an object of the present invention to effect both impedance transformation and transformation between single-ended and balanced circuits of arbitrary impedances while providing low-loss and very wide-bandwidth.
It is an object of the present invention to provide very wide-bandwidth matching between two arbitrary impedances.
Another object of the present invention is to provide highly-balanced performance over very wide bandwidth.
Another object of the present invention is to provide both arbitrary impedance matching and highly balanced single-ended-to-balanced operation over very wide-bandwidth.
Another object of the present invention is to provide, with low loss, wide-bandwidth, multiple identical output signals from a single source.
Another object of the present invention is to provide precision, low-loss, wide-bandwidth power division.
Another object of the present invention is to combine, with low loss and wide-bandwidth, multiple input signals to a single output signal.
Another object of the present invention is to simplify construction of RF impedance transformation devices by application of commonly available materials in novel constructions.
Another object of the present invention is to provide means to utilize various transmission-line structures to effect both transformation between two arbitrary impedances and transformation between two single-ended circuits.
Another object of the present invention is to provide means to utilize various transmission-line structures to effect both transformation between two arbitrary impedances and transformation between single-ended and balanced circuits.
Still another object of the present invention is to provide means to utilize various transmission-line structures to effect both transformation between two arbitrary impedances and transformation between single-ended and floating circuits.
Additional objects and advantages of the present invention in part will be set forth from the description that follows and in part from the description or learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by the methods and apparatus particularly pointed out in the appended claims.
It is a further object of the Wide-Bandwidth Balanced Transformer of the present invention to overcome the deficiencies of the devices of the prior art such as taught by Yusaku.
It is a further object of the Wide-Bandwidth Balanced Transformer invention to overcome the deficiencies of the devices of the prior art such as taught by Buschbeck.
It is a further object of the Wide-Bandwidth Balanced Transformer invention to overcome the deficiencies of the devices of the prior art such as taught by Guanella.
It is a further object of the Wide-Bandwidth Balanced Transformer invention to overcome the deficiencies of the devices of the prior art such as taught by Sevick.